Motion sensors and accelerometers are widely used in aerospace and automotive safety control systems and navigational systems, such as crash sensing systems. Automotive applications include anti-lock braking systems, active suspension systems, theft deterrent systems, supplemental inflatable restraint systems such as air bags, and seat belt lock-up systems. An example of a type of motion sensor employed in automotive systems is an acceleration sensor, or accelerometer, which senses acceleration, or more accurately, a force resulting from a change in the velocity of the vehicle. As with many motion sensors, an accelerometer operates on the basis of a moving body possessing inertia which tends to resist a change in velocity.
In the past, various types of electromechanical and electronic accelerometers have been used in the automotive industry to detect a change in an automobile's velocity. One type that has found wide acceptance in the industry is the piezoresistive accelerometer. Such accelerometers are generally composed of a silicon chip that has been micromachined to form a proof mass supported by one or more beams or bridges. Piezoresistors are formed on the bridge or bridges, and utilize the anisotropic piezoresistive characteristic of silicon to produce a change in resistance in response to strain induced in the bridges by the deflection of the proof mass. Piezoresistive accelerometers have a high transduction scale factor and offer a high degree of precision. Furthermore, they can be fabricated in a manner that is compatible with integrated circuit processing techniques, and are therefore widely used in the automotive industry.
The piezoresistors must be wired to suitable circuitry in order for their output to be employed to indicate acceleration. A conventional circuit for this purpose is a Wheatstone bridge. Four identical piezoresistors are typically used, with each piezoresistor forming a different leg of the bridge. In this fashion, the voltage measured across each resistor in each leg is half of the power supply to the bridge, and the differential between the two sides of the bridge, serving as the output of the circuit, is zero. Deflections of the proof mass are sensed by a change in resistance of the piezoresistors due to strains in the one or more bridges supporting the proof mass, causing a differential voltage signal to be present at the output of the Wheatstone bridge circuit. This output is proportional to the acceleration forces on the proof mass.
With the above design, it is often desirable or necessary to provide physical damping of the proof mass in order to achieve a damped electrical response to acceleration over frequency. A basic damping technique is squeeze film air damping, which is accomplished by creating a sealed cavity surrounding the proof mass with two or more additional chips that sandwich the proof mass chip.
Another type of accelerometer that has found acceptance in the automotive industry is the capacitive accelerometer, whose transduction method is that of a differential capacitor. Such accelerometers are generally formed by bulk etching a silicon chip, as noted above for piezoresistive accelerometers, or by surface micromachining techniques. In this design, a proof mass is supported by one or more bridges so as to be equidistant between a pair of capacitor electrodes when at a null position. These electrodes are typically formed by upper and lower capping chips that are bonded to opposite sides of the proof mass chip. The proof mass forms a third electrode, such that any deflection of the proof mass causes a change in the differential capacitance between the proof mass and the other two other electrodes, yielding an accelerometer output that is proportional to the acceleration forces on the proof mass. As with the piezoresistive accelerometer, the upper and lower capping chips can be used to form a sealed cavity around the proof mass so as to simultaneously provide squeeze film air damping for the proof mass.
Both the piezoresistive and capacitive accelerometers described above operate in an open loop manner, in that their response to acceleration produces an output that is directly measured in order to determine an acceleration force. A disadvantage with such accelerometers is that each employs a proof mass that must be deflected in order to produce an output, and therefore are often constructed so as to provide squeeze film air damping for the proof mass.
A solution to the above has been to further limit the movement of the proof mass within the cavity using a force rebalance technique which virtually prevents movement of the proof mass. This technique, represented in FIG. 1, has been employed with capacitive accelerometers, and utilizes an appropriate feedback circuitry 36 to detect the movement, indicated by arrows 32, of a proof mass 20 that is cantilevered from a frame 18 of the accelerometer 10. In the example, movement of the proof mass 20 toward an upper electrode 28a and away from a lower electrode 28b causes a change in capacitance between the proof mass 20 and the opposing electrodes 28a and 28b, causing a differential capacitance between the proof mass 20 and the electrodes 28a and 28b. The feedback circuitry 36 then applies a potential difference between the proof mass 20 and the electrode 28b, thereby creating an electrostatic force 30 that forces the proof mass 20 back to its null position. In this manner, the voltage-required to return and maintain the proof mass 20 at its null position is proportional to the acceleration force on the proof mass 20, and therefore serves as the output to the accelerometer 10.
A major advantage with this approach is that squeeze film air damping is not necessary since the proof mass 20 essentially does not move. As such, the cavity 24 need not be sealed, but can be vented to atmosphere. Another benefit is that, since the proof mass 20 does not move, the bridges supporting the proof mass 20 are not susceptible to fatigue. Finally, the use of the electronic feedback circuitry 36 can provide for a frequency response that is independent of the geometry of the accelerometer 10 and its proof mass 20, and that can be tailored to a specific application range.
Though the above noted advantages are significant, certain difficulties exist with the force rebalance accelerometers of the type described above. A primary difficulty is in fabricating a sensor that requires the capability of electrostatic deflection on both sides of the proof mass 20, necessitating electrical contacts to all three chips 12, 14 and 16. Secondly, the requirement for three separate chips 12, 14 and 16 increases the package size of the accelerometer 10. Another significant drawback is that the transduction scale factor for a capacitive sensor is relative low. As such, the feedback circuitry 36 for the capacitive accelerometer 10 must be provided on the same chip as the accelerometer 10 in order to avoid the use of connections that create parasitic capacitance, which would add to the noise of the system and degrade the output of the accelerometer. Such a requirement adds significant costs related to complex processing and higher yield losses, which are highly undesirable if the accelerometer is to be mass-produced. Another notable drawback is that the gap widths between the proof mass 20 and the electrodes 28a and 28b must be controlled to precise tolerances, since differences in gap widths will effect the operation of the accelerometer and its feedback circuitry 36.
Therefore, what is needed is a motion sensor that has the advantageous operational characteristics of a force rebalance motion sensor, but does not have the processing disadvantages or noise susceptibility of capacitive force rebalance accelerometers. Specifically, it would be desirable if such an accelerometer had a proof mass that is virtually immobilized in a manner that does not require squeeze film air damping or electrostatic balancing on both sides of the proof mass, were capable of fabrication with less than three separate electrical contacts and three chips, and enabled its control circuitry to be provided on a chip separate from the accelerometer.